There are numerous applications in which it is desired to form layers over substrates. For instance, it is frequently desired to form layers over semiconductor constructions during fabrication of integrated circuitry. Among the methods commonly utilized for layer formation are chemical vapor deposition (CVD) processes and atomic layer deposition (ALD) processes.
ALD technology typically involves formation of successive atomic layers on a substrate. Such layers may comprise, for example, an epitaxial, polycrystalline, and/or amorphous material. ALD may also be referred to as atomic layer epitaxy, atomic layer processing, etc.
Described in summary, ALD includes exposing an initial substrate to a first chemical species to accomplish chemisorption of the species onto the substrate. Theoretically, the chemisorption forms a monolayer that is uniformly one atom or molecule thick on the entire exposed initial substrate. In other words, a saturated monolayer. Practically, as further described below, chemisorption might not occur on all portions of the substrate. Nevertheless, such an imperfect monolayer is still a monolayer in the context of this document. In many applications, merely a substantially saturated monolayer may be suitable. A substantially saturated monolayer is one that will still yield a deposited layer exhibiting the quality and/or properties desired for such layer.
The first species is purged from over the substrate and a second chemical species is provided to chemisorb onto the first monolayer of the first species. The second species is then purged and the steps are repeated with exposure of the second species monolayer to the first species. In some cases, the two monolayers may be of the same species. Also, a third species or more may be successively chemisorbed and purged just as described for the first and second species. It is noted that one or more of the first, second and third species can be mixed with inert gas to speed up pressure saturation within a reaction chamber.
Purging may involve a variety of techniques including, but not limited to, contacting the substrate and/or monolayer with a carrier gas and/or lowering pressure to below the deposition pressure to reduce the concentration of a species contacting the substrate and/or chemisorbed species. Examples of carrier gases include N2, Ar, He, Ne, Kr, Xe, etc. Purging may instead include contacting the substrate and/or monolayer with any substance that allows chemisorption byproducts to desorb and reduces the concentration of a species preparatory to introducing another species. Purging time may be successively reduced to a purge time that yields an increase in film growth rate. The increase in film growth rate might be an indication of a change to a non-ALD process regime and may be used to establish a purge time limit.
ALD is often described as a self-limiting process, in that a finite number of sites exist on a substrate to which the first species may form chemical bonds. The second species might only bond to the first species and thus may also be self-limiting. Once all of the finite number of sites on a substrate are bonded with a first species, the first species will often not bond to other of the first species already bonded with the substrate. However, process conditions can be varied in ALD to promote such bonding and render ALD not self-limiting. Accordingly, ALD may also encompass a species forming other than one monolayer at a time by stacking of a species, forming a layer more than one atom or molecule thick. The various aspects of the present invention described herein are applicable to any circumstance where ALD may be desired. It is further noted that local chemical reactions can occur during ALD (for instance, an incoming reactant molecule can displace a molecule from an existing surface rather than forming a monolayer over the surface). To the extent that such chemical reactions occur, they are generally confined within the uppermost monolayer of a surface.
The general technology of chemical vapor deposition (CVD) includes a variety of more specific processes, including, but not limited to, plasma enhanced CVD and others. CVD is commonly used to form non-selectively a complete, deposited material on a substrate. One characteristic of CVD is the simultaneous presence of multiple species in the deposition chamber that react to form the deposited material. Such condition is contrasted with the purging criteria for traditional ALD wherein a substrate is contacted with a single deposition species that chemisorbs to a substrate or previously deposited species. An ALD process regime may provide a simultaneously contacted plurality of species of a type or under conditions such that ALD chemisorption, rather than CVD reaction occurs. Instead of reacting together, the species may chemisorb to a substrate or previously deposited species, providing a surface onto which subsequent species may next chemisorb to form a complete layer of desired material.
Under most CVD conditions, deposition occurs largely independent of the composition or surface properties of an underlying substrate. By contrast, chemisorption rate in ALD might be influenced by the composition, crystalline structure, and other properties of a substrate or chemisorbed species. Other process conditions, for example, pressure and temperature, may also influence chemisorption rate. Accordingly, observation indicates that chemisorption might not occur appreciably on particular portions of a substrate even though it occurs at a suitable rate on other portions of the same substrate.
A problem which can occur with CVD processes is that there is frequently less than 100% step coverage. ALD processes can frequently improve step coverage over CVD processes, but several difficulties are encountered during utilization of ALD processes.
One of the difficulties associated with ALD can occur in attempting to deliver sufficient flux of precursor within a reaction chamber for suitable step coverage and uniformity. The difficulty can be particularly severe when utilizing low vapor pressure precursor materials (such as, for example, materials volatilized from solid sources), with low vapor pressure precursor materials typically being understood to be materials having a vapor pressure of less than or equal to about 0.1 Torr at 100° C. Exemplary low vapor pressure precursor materials include HfCl4, TaF5, and pentakis(dimethylamino)tantalum (PDMAT).
Other difficulties encountered in ALD include, for example, difficulties associated with the formation of mixed-material films (sometimes referred to as doped films). For instance, it can be desired to form titanium-doped tantalum pentoxide (Ta2O5) or aluminum-doped hafnium oxide (HfO2). However, it can be difficult, and often seemingly impossible, to form a homogeneous film comprising low dopant levels during the monolayer-by-monolayer deposition of an ALD process. For instance, it can be desired for titanium-doped Ta2O5 to have about 8% TiO2 incorporated within a Ta2O5 matrix. Such can theoretically be accomplished by providing about twenty pulses of a tantalum precursor to one pulse of a titanium precursor during an ALD process. However, the material resulting from such process will typically have an atomic layer of TiO2 sandwiched between thick Ta2O5 layers, and often the TiO2 atomic layer will not even be continuous. Accordingly, the film resulting from separate pulses of titanium and tantalum in an ALD process is not the desired homogeneous mixture of TiO2 and Ta2O5. Thus, it is desired to develop new approaches for forming mixed materials utilizing ALD processes.
Although the invention was motivated at least in part by the difficulties discussed above relative to ALD processes, it is to be understood that the invention has applications beyond addressing such difficulties. The invention is therefore not to be limited to the addressing of such difficulties, or even to ALD processes, except to the extent that such limitations are expressly recited in the claims that follow.